The invention is of particular application to the analysis of low volume liquid samples, for example of volumes of five microlitres or less, such as would be used in the quantitative analysis of aqueous samples of DNA created in the laboratory. Since such DNA is a valuable resource, only a very small sample is usually available for assessment.
Typically, the analysis of a DNA sample involves passing a beam of light through the sample and measuring the amount of light absorbed at different wavelengths. Since the DNA has different absorbence characteristics from other proteins, the measurements can be used to provide a ratio of the concentration of DNA to that of other proteins in the sample.
U.S. Pat. No. 6,628,382 shows apparatus in which a sample in the form of a small volume liquid drop is sandwiched between two opposed anvil surfaces. The surfaces are moved together to compress the drop, at which point a first measurement is taken by passing light through the drop. The surfaces are then moved apart so as to draw the drop into a thin, concaved column in which condition a further measurement is taken by passing light axially along the column. After the measurements have been taken, the droplet cannot be fully retrieved since it will leave residues on the anvil surfaces which will need to be thoroughly cleaned in order to avoid problems of cross contamination with droplets subsequently placed in the apparatus. In addition, any dust in the sample droplet will affect the accuracy of the apparatus and the droplet, when drawn into the column, can lose significant amounts of liquid through evaporation and be affected by photobleaching caused by the ultraviolet light used in the measurement. These factors also affect the accuracy of the apparatus.
The invention seeks to avoid or at least mitigate one or more of the above-mentioned problems.